The present invention relates to a method for the manufacture of bundles of metal sheets of annular shape for stators of electric machines from stamped sheets.
It is known, for constructing stators of small and medium electric machines and, in particular, of alternators of motor vehicles, to form bundles of rings cut out by stamping in magnetic sheets or the like and to assemble them by riveting or welding.
The cut-out rings have, toward the inside, open slots formed by arms which are directed radially inward and which in the bundle will form the pole pieces of the stator. Cutting out an entire ring by stamping has some disadvantages: there is a large amount of waste, since that part of the sheet which corresponds to the inside diameter of the ring can be used only for forming a ring of smaller diameter, and indeed only insofar as the thickness of the sheet and the diameter correspond to the technical characteristics required. On the other hand, there is also a large amount of sheet waste corresponding to that part of the sheet which is outside the diameter of the ring.
The construction of the rings does not make it possible to maintain the required shape tolerances and the exact dimensions both of the rings and of the ring bundles on account of the internal stresses which are generated during stamping and which cannot easily be eliminated because of the closed shape of the ring. The internal stresses have an influence on the size tolerances of the inside and outside diameters of the rings and on the parallelism of two faces of a sheet bundle. It is often necessary to carry out additional machining of the stator before the required tolerances can be adhered to.
The Applicant's U.S. Pat. No. 4,597,172, the content of which is incorporated by reference, proposed a method and an apparatus for the manufacture of bundles of metal sheets from ring segments, in particular ring segments at 120°, without this being a limiting value.
This method and apparatus are satisfactory and make it possible, on the one hand, to make savings with regard to the sheet waste, since only ring segments are cut out and, on the other hand, to adhere more closely to the geometry of the bundle in that the internal stresses generated during the stamping of a closed ring are eliminated.
U.S. Pat. No. 4,102,040, the content of which is incorporated by reference, proposes a manufacturing method which, according to one variant, involves bending the sheet bundle formed by causing it to mate with the lateral surface of a drum in a continuous operation. When the two ends of the bundle meet, they are held by means of a connecting device and welding is then carried out.
U.S. Pat. No. 4,198,743, the content of which is incorporated by reference, describes a method which, according to one variant, makes it possible to form a C-shaped magnetic core by the middle of a sheet bundle being immobilized and by two simultaneously gripping tools being employed to exert, from the center, continuous forces making it possible to obtain the C-shape.